1. Field of the Invention
The present invention relates generally to an optical apparatus for synthesizing images, more particularly, is concerned with an image synthesizing apparatus for use in liquid crystal projectors and the like.
2. Related Background Art
There are conventionally known full-color liquid crystal projectors using three liquid crystal displays. In such liquid crystal projectors, white light from a light source is separated into three primary color beams of red light, blue light, and green light by a color separation optical system, and the color beams are guided as illumination light into respective corresponding displays. Each color beam incident into the corresponding display is modulated according to an image signal and the modulated beam is outputted. The color beams outputted from the respective displays are guided into a color beam combining optical system, e.g., a cross dichroic prism to be combined into one beam. The composite light formed in the cross dichroic prism is guided through a projection lens to be projected onto a screen, thereby forming an image on the screen. In the liquid crystal projectors as described above, the three liquid crystal displays are supported by a housing as a support with respect to the cross dichroic prism (Japanese Patent Application Laid-Open No. H06-14326).
In the case where the three liquid crystal displays are supported by the housing as described above, however, if the housing undergoes thermal expansion or thermal contraction with change in temperature, for example, so as to change only the distance between the cross dichroic prism and one display, there will arise a problem that the color beam outputted from the liquid crystal display fails to focus on the screen and the image formed by the color beam is out of focus.
Particularly, the above-mentioned problem becomes significant, for example, in reflective liquid crystal projectors with application of a so-called OFF-AXIS type technique of beam incidence into reflective liquid crystal displays in which the color beams are obliquely incident into the reflective liquid crystal displays (National Publication of Translated Version of PCT Application, Publication No. 2001-520403). This is because the cross dichroic prism has to be placed apart from the reflective liquid crystal displays in the case of the oblique incidence of the color beams into the reflective liquid crystal displays.
The present invention has been accomplished under the above circumstances and an object of the invention is to provide image synthesizing optical apparatus capable of preventing the failure in focus of an image of a specific color beam due to change in temperature.
In order to achieve the object, an image synthesizing optical apparatus according to the present invention is characterized by comprising: a first display for modulating a first color beam (e.g., red light) according to an input image signal and outputting the first color beam thus modulated; a second display for modulating a second color beam (e.g., blue light) according to an input image signal and outputting the second color beam thus modulated, the second display being arranged such that an optical axis of the second color beam outputted from the second display coincides with an optical axis of the first color beam outputted from the first display; a third display for modulating a third color beam (e.g., green light) according to an input image signal and outputting the third color beam thus modulated, the third display being arranged such that an optical axis of the third color beam outputted from the third display passes a midpoint between the first display and the second display, on the optical axis of the color beams outputted from the first display and from the second display; a color beam combining unit disposed at the midpoint and configured to combine the color beams outputted from the first display, from the second display, and from the third display; and a support for supporting the first display, the second display, the third display, and the color beam combining unit, wherein the support is configured such that optical path lengths of the color beams from the first display, from the second display, and from the third display to the color beam combining unit are kept equal to each other even with change in temperature.
With this arrangement, the color beams outputted from the first display, from the second display, and from the third display can travel in the same optical path length before entrance into the color beam combining unit in spite of change in temperature, and the color beam combining unit can combine those color beams into one beam.
The first display, second display, and third display each can be reflective displays. In the case of the reflective displays, distances become long between the displays and the color beam combining unit and are susceptible to change in temperature, such that the arrangement of the present invention can be effectively applicable.
Furthermore, the first display, second display, and third display can also be liquid crystal displays. The color beam combining unit can be one comprised of four rectangular prisms.
Preferably, the above-stated support comprises: a first support part for supporting the first display relative to the color beam combining unit; a second support part for supporting the second display relative to the color beam combining unit; and a third support part for supporting the third display relative to the color beam combining unit, wherein the first support part, the second support part, and the third support part are made of an identical material and in an identical shape. Since the support parts are made of the identical material and in the identical shape, the support parts similarly suffer influence of change in temperature. For this reason, there will never occur such an event that only one support part changes its length with change in temperature so as to result in changing only the optical path length of the color beam from the display supported by the support part, to the color beam combining unit.
Furthermore, preferably, the support having the first support part, second support part, and third support part further comprises a center support part for supporting the color beam combining unit, wherein the first support part, the second support part, and the third support part are arranged to extend radially from the center support part. Since the center support part supports the color beam combining unit, the optical path lengths from the displays supported by the support parts arranged to extend from the center support part and formed in the same shape, to the color combining unit similarly vary with change in temperature.
It is preferable that distances from a center axis of the center support part to the first display, to the second display, and to the third display are equal to each other.
These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.